Quad Flat No-lead (QFN) semiconductor chip packaging technology provides compact and reliable chip packages. The size of a semiconductor chip package, especially the height, is important in increasing the density of semiconductor chips in an electronic device. For example, conventional methods disclose a semiconductor package that comprises an external leadless leadframe package and an internal package, wherein the internal package is disposed within the cavity of the external leadless leadframe package, resulting in an increase of chip density within the semiconductor package.
In order for the internal package to be disposed within the cavity, the dimension of the internal package is minimized by having an exposed die on the package bottom surface and exposed leads on the package top and bottom surfaces. However, the exposed contact areas of the leads on the top and bottom package surfaces are not aligned in the package thickness direction due to one-sided recesses formed in the leads. This leadframe design has certain disadvantages such as warpage, variation, and delamination. Furthermore, the disclosed internal package lacks versatility of stacking two or more packages or stacking one package in two different directions. The reason for why the contact areas of the leads are offset may be due to the limitation of the method used to manufacture the leadframe. As is well known, the method of manufacturing this type of leadframe is using two photolithography masks for half-etching on the top and bottom surfaces of the raw leadframe material, so as to achieve the necessary recesses on the inner lead structure.
The semiconductor chip in a QFN package may be optical sensor die when the QFN package is an optical sensor. The typical Optical Quad Flat No-Lead (OQFN) packages known to those in the art include a die or dies settling in a molded cavity and a glass lid sitting on the top of the molded cavity so that lights can pass through the glass lid and then reach the die without interference. FIG. 1 illustrates a schematic cross-section of the current OQFN package 10 that has a leadframe providing inner leads 1 and a die-attach pad 2, a die attach adhesive layer 3, a die 4 being attached to the die-attach pad by the die attach adhesive layer 3, a molded wall 5 forming a molded cavity enabling the die to be settled within the cavity, a glass attach 6 forming at the top inner edge of the molded wall, and a glass lid 7 sitting on the top of the glass attach so that the molded cavity with the die is encapsulated. Because the die is exposed in the molded cavity, the lights can reach the die directly through the glass lid. However, the current OQFN package has certain disadvantages. Among them are thick package with a total package height over 1.5 mm, high tooling cost for requiring dedicated cavity mold for different sized OQFN packages, material cost for glass lid and attach, and additional process steps of glass lid attach.
Accordingly, what is needed is an ultra-thin Quad Flat No-Lead (QFN) semiconductor chip package and method of making the same, wherein the QFN chip packages have the versatility of being stacked in different directions or numbers. More specifically, what is needed is an ultra-thin optical QFN (OQFN) package that is thin and can be manufactured in less process steps than the known OQFN packages.